Some prior art hydraulic actuators, such as the actuators disclosed in WO 96/07029 or U.S. Pat. No. 4,870,892 provide safety measures in the case of electrical power cut off or actuator failure. In the actuators disclosed in WO 96/07029 and U.S. Pat. No. 4,870,892 this is obtained by causing a sliding member to move to a neutral position in the case of power cut off or actuator failure. In the actuator disclosed in U.S. Pat. No. 4,870,892 this is obtained by positioning the sliding member in the diagonal of a bridge circuit. Two valves which are normally closed in a de-energized state are fluidly connected between a pump and two pressure chambers of the sliding member, and two valves which are normally open in a de-energized state are fluidly connected between the pressure chambers and a tank. Furthermore, two biasing springs are arranged in the sliding member, biasing the sliding member towards the neutral position. Thus, in the case of an electrical power cut off the two valves arranged on the pump side are closed and the two valves on the tank side are opened. In the absence of fluid pressure from the pump, due to the closed valves on the pump side, the biasing springs will push the sliding member towards the neutral position, and fluid is allowed to flow between the tank and the pressure chambers, due to the open valves on the tank side. Accordingly, the sliding member is moved into the neutral position.
In the actuator disclosed in WO 96/07029 a sliding member is also positioned in the diagonal of a bridge circuit. However, in this case all four valves are of the normally open type. Thus, in the case of a power cut off, all four valves are opened. Thereby there is a permanent flow of fluid from the pressure source (pump) to the pressure sink (tank). Since this flow of fluid is distributed uniformly over the two branches of the bridge circuit, the pressure on each side of the sliding member is the same. Thereby the sliding member will be moved to a neutral position.
However, in some hydraulic position motors, such as those incorporated in hydro-mechanical transmissions (HMT's) on all terrain vehicles or work utility vehicles, there is a need for locking a hydraulic servomotor in its instantaneous position, if a failure is detected or loss of power happens. For example, the driver must, for safety reasons, not be exerted to potential hazardous accelerations, and the gearing ratio of the HMT must therefore be maintained in the case of a power loss or actuator failure. With regard to the hydraulic servomotor, this means that it should be locked in its instantaneous position. In such applications it is therefore not necessarily appropriate that the hydraulic servomotor must move to a neutral position as described in WO 96/07029 and U.S. Pat. No. 4,870,892.
U.S. Pat. No. 4,416,187 discloses a servosystem having solenoid activated on-off valves governing the amount of fluid in two variable volume chambers of a double acting piston-cylinder unit. The servosystem may comprise a valve arrangement in which a check valve and a restriction are arranged between a pump and the first variable volume chamber, another check valve and a restriction are arranged between the pump and the second variable volume chamber, a solenoid valve which is normally closed in a de-energized state is arranged between the first variable volume chamber and a fluid drain, and another solenoid valve which is normally closed in a de-energized state is arranged between the second variable volume chamber and the fluid drain. The check valves are positioned in such a manner that a flow of fluid is allowed in a direction from the pump towards the fluid chambers. In the case of a power cut off the two valves arranged between the variable volume chambers and the fluid drain will be closed, and thereby fluid flow out of the variable volume chambers is prevented. As a consequence, the piston is not able to move in the cylinder, and the servosystem is therefore locked in its instantaneous position. However, in the servosystem of U.S. Pat. No. 4,416,187 the locking feature is only obtained if the valves arranged between the variable volume chambers and the fluid drain are both of the kind which is normally closed in a de-energized state. This limits the possibilities of designing the bridge circuit while taking other factors into consideration, and it may thereby not be possible to design the bridge circuit in a manner which optimizes the performance of the servosystem during normal operation.